全文获取类型
收费全文 | 22719篇 |
免费 | 3240篇 |
国内免费 | 2206篇 |
专业分类
电工技术 | 1364篇 |
综合类 | 1775篇 |
化学工业 | 1899篇 |
金属工艺 | 622篇 |
机械仪表 | 1548篇 |
建筑科学 | 414篇 |
矿业工程 | 294篇 |
能源动力 | 447篇 |
轻工业 | 1837篇 |
水利工程 | 73篇 |
石油天然气 | 584篇 |
武器工业 | 626篇 |
无线电 | 10185篇 |
一般工业技术 | 2198篇 |
冶金工业 | 373篇 |
原子能技术 | 202篇 |
自动化技术 | 3724篇 |
出版年
2024年 | 61篇 |
2023年 | 368篇 |
2022年 | 568篇 |
2021年 | 688篇 |
2020年 | 827篇 |
2019年 | 712篇 |
2018年 | 748篇 |
2017年 | 922篇 |
2016年 | 1035篇 |
2015年 | 1169篇 |
2014年 | 1591篇 |
2013年 | 1526篇 |
2012年 | 1991篇 |
2011年 | 2022篇 |
2010年 | 1388篇 |
2009年 | 1368篇 |
2008年 | 1445篇 |
2007年 | 1593篇 |
2006年 | 1442篇 |
2005年 | 1077篇 |
2004年 | 894篇 |
2003年 | 787篇 |
2002年 | 604篇 |
2001年 | 563篇 |
2000年 | 486篇 |
1999年 | 378篇 |
1998年 | 347篇 |
1997年 | 268篇 |
1996年 | 243篇 |
1995年 | 202篇 |
1994年 | 194篇 |
1993年 | 148篇 |
1992年 | 129篇 |
1991年 | 112篇 |
1990年 | 80篇 |
1989年 | 46篇 |
1988年 | 18篇 |
1987年 | 22篇 |
1986年 | 16篇 |
1985年 | 11篇 |
1984年 | 19篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1973年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 968 毫秒
91.
This study shows the connection between the morphological structure of transferred films and the chemical composition of the monolayer before transfer. The stability of a behenic acid monolayer depends on the linked influence of the type of cation and of the pH value in the aqueous subphase. The stability of the monolayer is assessed by the expansion of the solid-packed state and by the relaxation time. Its integrity is characterized by the number of crystal defects which appear in transferred Langmuir-Blodgett (LB) films as seen through Nomarski microscope observations. Both stability and integrity are greatly improved when Cd2+ or Mn2+are added to the subphase at pH 5.75 and 6 respectively. In these conditions, the transfer is very efficient and the LB films thus obtained are of very high quality. The pH values of the subphase at which the molecules of behenic acid are transformed into bivalent salts differ significantly according to the nature of the cation involved. Comparing infrared spectra of the LB films, it appeared that the relative amount of acid/salt found in the monolayer was practically the same with a subphase containing either Cd2+ at pH 5.75 or Mn2+ at pH 6. This suggests that the ratio of acidic and salt forms would be a key parameter for obtaining high-quality LB films. 相似文献
92.
In the power transformer, the presence of polar or charged species in the insulating oil can cause failure and electric discharges. Solid substrates such as silica can be used to extract the polar species and to refine the oil in order to prevent future failure in the power transformer. However, the use of silica for petroleum oil separation and refining will depend on the silica characteristics such as surface charge, surface composition, specific surface area and particle size.Various pyrogenic silicas having various specific surface areas (49-200 m2 g−1) and particle sizes (207-500 nm) were used to extract the polar fractions from the neat transformer insulating oils (a new, NO, and used, UO2, oils). The oil covered silica samples were investigated by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy in the range 4000-600 cm−1. The bare silica surfaces present two main hydroxyl components, a sharper peak at 3745 cm−1, I3745, due to isolated silanols and a second broad, which spreads over 3745-3000 cm−1, I3745-3000, due to H-bonded silanols. The relative intensities of the two components, I3745/I3745-3000, varied for the bare and the oil covered silicas depending on the solid surface characteristics. The adsorption of the NO polar fraction onto silica leads to strong reduction in intensity of the sharper peak in favour of the broad one. However, the adsorption of the UO2 polar fraction onto silica leads in all cases to the decrease in the intensity of the both silica OH components. Further, the UO2 adsorption on the silica leads to the apparition of a broad peak at low frequency in the region 3250-3300 cm−1 which, is due to the associated phenolic groups of UO2 oil polar fraction. The analysis of the DRIFT spectra for various samples indicates that the oil polar fraction resembles to asphaltenes compounds.The microelectrophoresis method used to investigate the surface charge at the water/oil polar fraction covered silica interface, indicates negatively charged particles. Further, the negative charge increased with the pH, as resulting from the increase of the ionisation and/or the amount of the oil polar carboxylic and phenolic groups. The oil polar fraction, i.e. the asphaltene components, in contact with both the silica surface and water at high pH values rearrange, due to their amphiphilic character.Finally, the use of the silica substrates seems to be suited to extract and analyse polar species present in petroleum oil. A correlation is found between the nature of the oil, its functionalities, and the magnitude of its zeta potential value at the water/oil covered silica interface. 相似文献
93.
94.
M. J. Vissenberg P. W. de Bont E. M. van Oers R. A. de Haan E. Boellaard A. M. van der Kraan V. H. J. de Beer R. A. van Santen 《Catalysis Letters》1996,40(1-2):25-29
Ion exchanged CoNaY was sulfided at 473 and 673 K and subsequently heated in He at 673 and 773 K. The resulting samples were characterized by means of overall sulfur analysis, temperature programmed Ar treatment and Fourier transform infrared spectroscopy. It was shown that during He flushing at sufficiently high temperature a protolysis reaction occurs resulting in the decomposition of Co sulfide into Co2+ ions and H2S. 相似文献
95.
Modified Imidazoles: Degradation Inhibitors and Adhesion Promoters for Polyimide Films on Copper Substrates 总被引:1,自引:0,他引:1
Polyimide films on copper substrates that are exposed to elevated temperatures and an oxidizing environment will be subject to degradation. In order to halt this degradation without changing the properties of the system, a polymeric agent could be placed between the polyimide and the copper. This paper will investigate three such materials that will not only slow down the degradation of the polyimide and the oxidation of the copper, but will also improve adhesion within the system. Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) will be used to investigate the polyimide/polymeric agent/copper system. 相似文献
96.
97.
Robot Pose Estimation in Unknown Environments by Matching 2D Range Scans 总被引:14,自引:0,他引:14
A mobile robot exploring an unknown environment has no absolute frame of reference for its position, other than features it detects through its sensors. Using distinguishable landmarks is one possible approach, but it requires solving the object recognition problem. In particular, when the robot uses two-dimensional laser range scans for localization, it is difficult to accurately detect and localize landmarks in the environment (such as corners and occlusions) from the range scans.In this paper, we develop two new iterative algorithms to register a range scan to a previous scan so as to compute relative robot positions in an unknown environment, that avoid the above problems. The first algorithm is based on matching data points with tangent directions in two scans and minimizing a distance function in order to solve the displacement between the scans. The second algorithm establishes correspondences between points in the two scans and then solves the point-to-point least-squares problem to compute the relative pose of the two scans. Our methods work in curved environments and can handle partial occlusions by rejecting outliers. 相似文献
98.
系统地研究了MgF2衬底上类金刚石薄膜的折射率和生长速率与淀积工艺之间的关系,在MgF2衬底上成功地设计并制备了红外增透和保护膜。理论与实验研究表明,类金刚石薄膜使MgF2的红外透过率提高3%以上是完全可靠的,但起红外增透和保护是作用的统一性问题仍有待进一步研究。 相似文献
99.
利用垂直阵列构作了一类Cartesian认证码,计算了它们的参数,并且在编码规则按等概率分布选取时,成功的模仿攻击和成功的替换攻击概率也被算出。 相似文献
100.
本文提出了针对递归DSP算法的高层次系统综合流程,并以脉动(systolic)式处理器阵列结构实现.从DSP算法的FDDL行为级描述开始,经由编译及划分,产生数据相关流图(Data Dependency Graph),然后实现对算法流图的空间映射及时域规划,得到算法的信号流图(Signal Flow Graph),经时序重构,生成脉动阵列,最后实现对处理器单元的数据路径综合及控制器综合,并对处理器单元定位,本文同时提出了各设计阶段的算法策略及优化策略,并给出综合结果。 相似文献